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General format. For a given precision p >= 1, this rounds the number

to p significant digits and then formats the result in either

fixed-point format or in scientific notation, depending on its

magnitude.

The precise rules are as follows: suppose that the result formatted

with presentation type 'e' and precision p-1 would have exponent exp.

Then if -4 <= exp < p, the number is formatted with presentation type

'f' and precision p-1-exp. Otherwise, the number is formatted with

presentation type 'e' and precision p-1. In both cases insignificant

trailing zeros are removed from the significand, and the decimal point

is also removed if there are no remaining digits following it.

Positive and negative infinity, positive and negative zero, and nans,

are formatted as inf, -inf, 0, -0 and nan respectively, regardless of

the precision.

A precision of 0 is treated as equivalent to a precision of 1. The

default precision is 6.